The present invention is directed to an improvement in apparatus known as shunt flowmeters which measure the flow of fluid through a main supply line. Typically, such flowmeters employ a by-pass, or shunt path, to divert a fraction of the flow in the main line. This shunt path incorporates a sensor which measures the fractional part of the flow in the shunt path to determine the total flow in the main supply line.
The sensor which is employed typically comprises a heated length of capillary-like tubing supported at its ends by heat sinks and having one or more thermocouple junctions intermediate its ends. The temperature sensed at the thermocouple junction(s) is a function of the rate of flow of fluid through the capillary. Examples of such sensors are disclosed in U.S. Pat. Nos. 3,181,357 and 3,229,522, issued respectively on May 4, 1965 and Jan. 18, 1966 to James M. Benson, and in U.S. Pat. Nos. 4,245,503 and 4,270,386 issued respectively on Jan. 20, 1981 and June 2, 1981 to Charles E. Hawk, et al.
These flowmeters are intended to measure the amount of heat transferred to the gas stream, and as such are mass flowmeters. The (M) mass of a gas passing through the heated conduit is proportional to the (H) heat input divided by the (Cp) heat capacity and the change in temperature (.DELTA.T) of the gas, i.e., M=H/Cp .DELTA.T. Since heat capacity is very stable with changes in static pressure and temperature, the relationship is not affected by these parameters over normally encountered limits.
When sensors of the type just described are used in shunt flowmeters, the main line and the shunt characteristics must be such that the branching ratio remains constant, independent of pressure or temperature variations. This is accomplished by constructing a laminar flow "obstruction" in the main line to divert flow through the sensors which are approximately identical in diameter and length to the sensor conduit. The main line is constructed of a multiple of these conduits such that the total flow is directly related to the sensor conduit by the ratio of the number of conduits in the main line.
Under conditions wherein the flowing fluid being measured is at a temperature less than about 50.degree. C., the sensor, which is calibrated to normal ambient temperature, operates in a predictable fashion. However, since the sensor is positioned close to the main line by relatively short conduit sections, the passage of high temperature fluid through the main line and the shunt path exposes the sensor to temperature conditions which affect its reliability. For example, the resistances of the heated tubing and its heating devices increase, and the thermal conductivity of the gas surrounding the heated conduit varies. As a result, the temperature gradients along the heated capillary are no longer the same as when the sensor was calibrated at room temperature, and the fluid flow measurements consequently contain substantial error. Due to the long time periods required to reach temperature equilibrium, recalibration of the sensor for higher than ambient temperatures is impractical, and similarly, conventional devices are useless under conditions of rapid high temperature changes.